Some fact about oracle listener, tns, service & SID

1. Service_Name and SID are two different things. Service name directly maps to service of database. SID is system identification. The difference will be apparent in RAC environment where SID will be instance name and service name will be database service. This difference will also come into picture if your service name contains domain name. For example, your service name is dw.tmib.net.bd but your SID is dw.
   
   
2. For OS authentication, ORACLE_SID should be set to instance name i.e. SID.
   
   
3. For Listener, SID and Service Name are interoperable.
   
   
4. tnsping command confirms that there is a listener which listens to host:port referred by the tns entry.
   
   
5. In sqlplus connect command, @tnsname refers to tns entry in tnsnames.ora file. tns entry always goes to listener.
   
   
6. If the listener is not running, from sqlplus of server the database can be still connected. In this case, it is recommended to set the ORACLE_SID variable. It is required not to use @tnsname.
   
   
7. In a tns entry, under the description several addresses can be added. This helps the fail over. If any host:port service fails then go to the next host:port.
   DW=
   (DESCRIPTION=
   (ADDRESS=
   (PROTOCOL=TCP)
   (HOST=192.168.8.21)
   (PORT=1521)
   )
   (ADDRESS=
   (PROTOCOL=TCP)
   (HOST=192.168.8.22)
   (PORT=1521)
   )
   (CONNECT_DATA=
   (SERVER=dedicated)
   (SID=dw2)
   )
   )
   So if 192.168.8.22:1521 fails to serve, the request will be forwarded to 192.168.8.1:1521